The CCNet resource pack includes custom armour textures that can be activated by renaming armour items in a certain way.
To see these textures, you need to install Optifine or the CITResewn mod for Fabric.
American for the USAAustro for the Austro-Hungarian EmpireBelgian for BelgiumBritish for BritainFrench for FranceGerman for the German EmpireJapanese for JapanOttoman for the Ottoman EmpireRussian for the Russian EmpireItalian for the Kingdom of ItalyFinnish for FinlandCultistIronCultistGoldCultistDiamondCultistNetheriteCultist
You can change the texture of leather horse armour by dyeing it a specific colour. Unlike player armour, this does not require Optifine or CITResewn - the CCNet resource pack itself is sufficient.